Cyclospora spreads through human waste – so how did it get on food?

The is very likely a in the food supply, and .

The parasite’s spread can become a self-perpetuating cycle. Cyclospora is found only in humans, where it replicates in the intestinal tract, causing severe diarrhea. The organism reproduces by releasing oocysts, which are . In the U.S., feces usually end up in sewage systems.

In many states, sewage water is treated – though not always in ways that remove or kill the oocysts – and then . The oocysts mature in the environment with warm temperatures in about a week. Then a human comes along and eats the irrigated food or drinks the water and gets sick, releasing more oocysts into sewage systems.

I am a who collects sewage water and tests it for evidence of harmful pathogens, such as cyclospora. The first time I studied cyclospora was in the first outbreak ever documented in the U.S., a in 1995.

The cause was initially suspected to be strawberries grown in California, but later it was determined it was more likely due to imported . That country’s raspberries were also associated with much larger outbreaks in 1996 and 1997. At the time, little was known about this .

More than 30 years later, the largest outbreak ever recorded in the U.S. has , and , where I live and work. Using is common in many places, especially where groundwater and surface water are in short supply.

What’s in wastewater

There are two reasons researchers like me collect sewage and monitor what’s in it.

The first is to evaluate levels of disease in a community. Measuring viruses such as helped paint a picture of the spread of COVID-19 and the emergence of variants as the pandemic continued.

The second reason is to determine whether and how sewage treatment removes, inactivates or kills pathogens.

It is not easy to measure cyclospora oocysts in sewage, contaminated water or food. Even have trouble reliably detecting low levels of oocysts, which can still cause disease.

Cyclospora oocysts have been found in sewage around the world. A range of studies across the world shows that they can be detected in – but not all studies report how high or low the concentrations of the oocysts were. So it can be hard to say exactly how widespread it is.

A person who is infected with cyclospora excretes for as long as 60 days. Based on what is known about other fecal pathogen excretions from patients, related to the concentrations of those , I estimate that there could be anywhere from 1 to 100 oocysts per liter in sewage.

Our laboratory at Michigan State University is developing a method to more accurately detect this parasite even at lower levels in sewage. This type of wastewater surveillance may help determine when an outbreak is beginning to subside and where larger numbers of people are still affected. In addition, this information could allow sewage treatment plant managers to regularly monitor their discharges.

What happens during sewage treatment?

There isn’t clear data on how well standard sewage treatment processes reduce the numbers of . But there is information on two other similar protozoa that also cause significant diarrhea in humans: produces an oocyst about half the size of cyclospora, and produces a cyst that is similar in size to cyclospora.

From 2001 to 2003, my laboratory studied the occurrence of those protozoa at six sewage treatment plants – one each in Arizona and California and four in Florida. All had been approved by their respective state regulators for nonpotable reuse, including irrigation of landscapes and, in some cases, crops.

In that study, we found these protozoa in all the untreated sewage we tested, indicating there is some portion of the population that is infected and excreting their cysts or oocysts.

The five treatment plants whose processes included disinfection with chlorine were able to get rid of a high percentage – but not all – of the protozoa in their discharged wastewater. Some , with the potential to cause disease, even in water that had gone through the entire treatment process, as it is known that wastewater chlorination does not kill these protozoa.

From that data, it seems reasonable to assume that at least some small proportion of cyclospora oocysts also survive sewage treatment processes and are released back into the environment, where they can survive for months.

What about reuse?

Across the U.S., are used for irrigation of agricultural lands each year.

Some of that wastewater undergoes for reuse before being spread directly on landscapes or crops. The volumes, however, are not readily known. And other treated sewage, which undergoes only standard secondary treatment, is discharged into rivers, streams and reservoirs that could provide irrigation water.

But to remove or monitor protozoa such as cyclospora in treated sewage. Filtration can remove protozoa but must be designed and operated correctly. Chlorination is not effective, but and eimeria, a chicken protozoan highly related to and used as a by the food safety industry.

What is needed for better risk assessment and management?

As floods and droughts occur routinely across the U.S. and around the world, sewage may or be used directly on crops. That increases the risk that diseases, including protozoa such as cyclospora, which are transmitted through ingestion of fecal matter, may spread through food and water supplies.

And because , it is possible that increasing temperatures, such as heat domes, could speed up their maturation, exposing more people to the infectious form of the parasite.

Modern technology and techniques are capable of monitoring water quality, detecting harmful pathogens and eliminating them. Expanding monitoring of wastewater for protozoan diseases and other dangers can help prevent outbreaks and slow the spread of disease from sewage-contaminated water in the future.